The invention is directed toward multistage vertical tray-type vapor-liquid contactors which find use in a variety of equipment and industrial processes such as fractional distillation, absorption, stripping, mixing, and partial condensation (dephlegmation).
Multicomponent fluid vapor-liquid contactors of the tray or plate type have several known limitations which result in large diameter, large height, high cost, high-energy demand, and high-pressure drop. The diameter is determined by the flooding limitation, i.e., the loading. The overall height is determined by the tray efficiency and height of a tray. Tray height is related to the point efficiency and froth height, which are interrelated, and are controlled by vapor injector geometry and weir height. Tray pressure drop is comprised of the vapor injector drop plus the tray liquid head. The energy demand is determined by the reboil requirement for above-ambient contactors, and by the reflux requirement for below-ambient contactors. Incorporating heat exchange in the column (i.e., diabatic distillation) is known to be one method of reducing the external reboil or reflux demand. Examples of diabatic distillation are disclosed in U.S. Pat. Nos. 385,504; 2,330,326; 2,492,932; 2,963,872; and 3,642,452. The limitations enumerated above are interrelated to some extent, with various tradeoffs possible. For example, the energy requirement can be reduced somewhat by adding trays and hence increasing column height, or vice versa. Larger diameter can reduce tray height and overall height, etc.
Vapor-liquid contactors are used in distillation columns, rectifiers, strippers, absorbers, mixing (reverse distillation) columns, absorption cycle apparatus, bioreactors, reactive distillation columns, and similar devices.
Various approaches to overcoming the above limitations have been disclosed in the prior art. Higher point efficiencies have been achieved in spinning cone contactors, and by imparting horizontal velocity to part of the vapor. Higher vapor loadings have been achieved using locally co-current trays plus separators which route the liquid to a twice-lower tray (U.S. Pat. No. 4,361,469). Other approaches to achieving local cocurrency are disclosed in U.S. Pat. Nos. 2,693,350; 3,642;542; 5,766,519; and 5,798,086. Conventional trays have a crosscurrent contact pattern, and cannot be increased to the higher loading characteristic of cocurrency without flooding. Higher liquid loadings have been achieved by multiple downcomers (U.S. Pat. No. 3,410,540). Reduced energy demand has been achieved in the diabatic distillation disclosures. One problem with the prior art approaches to overcoming the various limitations listed above is that they usually address only a single limitation, with modifications which may make it more difficult to overcome the other limitations.
It is known that conventional crosscurrent trays approach a tray efficiency limit of 150% of the point efficiency when the horizontal liquid flow direction is reversed on each successive tray and the vapor is unmixed; 167% with fully mixed vapor; and 200% when the liquid direction is the same on each tray and the vapor is unmixed. This boost in efficiency is owing to the extra boost provided by effectively having multiple horizontal stages on each vertical tray, i.e., due to concentration gradients across the tray.
What is needed, and among the objects of this invention, are apparatus and process for vapor-liquid contact which achieve the thermodynamic advantage of globally counter-current multistage contact, the tray efficiency (reduced column height) advantage of tray crosscurrent contact, and the point efficiency and loading (reduced column diameter) advantages of locally co-current liquid recirculated contact. Preferably, the contactor would achieve further increase in tray efficiency and corresponding decrease in tray count and column height by same-direction horizontal liquid flow on each tray coupled with structure to prevent vapor mixing. Preferably, the contactor would also achieve lower energy demand. Most preferably, all of those desirable objects would be achieved simultaneously by compatible and synergistic measures.
The above and additional useful advantages are obtained in apparatus and corresponding process for multistage multicomponent fluid mass exchange wherein at least one type of same-direction liquid flow is present on all the trays of the contact section: horizontal same-direction flow, and/or vertical same-direction. More particularly, in the latter case, the contactor is comprised of:
A vapor-liquid contactor comprised of:
a. a multiplicity of vertically stacked trays within a containment;
b. a multiplicity of channel weirs on each tray, each channel weir defining a locally co-current vapor-liquid upflow zone on one side of the channel weir, and a liquid down flow zone on the other side; said channel weirs having a liquid passage opening at or near the bottom for transport of liquid from the downflow zone to the co-current upflow zone;
c. a multiplicity of vapor passages through said trays at the bottom of said co-current upflow zones;
d. a level control liquid weir on each tray, which is at a lower height than said channel weirs;
e. a passage for transport of liquid spillover from said liquid weir to the next lower tray;
f. a vapor supply below said stack of trays; and
g. a liquid supply and vapor withdrawal above said stack of trays
In contrast to conventional trays which have a turbulent froth region wherein the liquid flows in all directions, the above disclosure results in liquid flowing vertically the same direction (downward) in all the downcomers, and upward in all the risers (upflow zones).
More intensified vapor-liquid contact, and hence higher point efficiency or lower froth height, can be obtained by placing enhanced contact media in the co-current upflow channels. The required amount of media is relatively very small, since much of the tray volume is empty space, yet all the fluid traverses the upflow channels, which are highly loaded. The media can also be catalytically active, to support chemically reactive distillation. Thus maximum utilization of a small amount of catalyst is achieved.
The tray loading can be further increased, and/or tray height further decreased, by providing apparatus for vapor-liquid separation above the channel weirs. Thus the vapor-liquid separation is accelerated and made more effective compared to simply relying on open-space separation. It is important that the liquid drainage from the separator be directed to the downflow zone (downcomer channels), as otherwise it can be re-entrained in the upflowing vapor.
Each tray is fitted with a level control liquid weir, much like conventional trays, and the weir overflow is routed to the next lower tray. It is frequently preferred to route the overflow liquid to the same comparative location on each succeeding or adjoining trayxe2x80x94this allows the tray efficiency to approach 200% of the point efficiency provided the vapor is prevented from mixing, e.g., by partitions. When the horizontal liquid flow direction is opposite on adjacent trays, then the tray efficiency only approaches 150% of the point efficiency for unmixed vapor, or 167% for fully mixed vapor.
This gives rise to the other case of same direction liquid flowxe2x80x94horizontal. In that case, the contactor is comprised of:
a. a multiplicity of vertically stacked trays within a containment;
b. a liquid supply area and liquid removal area to and from each tray, wherein each supply and removal area is at the same relative location on each tray, whereby the net horizontal liquid flow on each tray is in the same directional pattern; and
c. a multiplicity of partitions in the vapor space of each tray which are transverse to the flow direction of said liquid, and thereby minimize vapor mixing.
The compartmentalization reduces the horizontal mixing of both the liquid and the vapor, resulting in larger concentration gradients and hence higher tray efficiencies for a given point efficiency (provided the horizontal liquid flow is the same direction on each corresponding section of each tray). For very large diameter trays, the partitions only need to be in the vapor space; for smaller trays, they should extend into the liquid space as well to reduce liquid mixing. When they do, suitable openings or clearance for liquid transport are provided.
In order to ensure good liquid renewal on all active parts of the tray, the liquid transport openings in the respective compartment partitions can be staggered so as to ensure a tortuous liquid flowpath across the entire tray. The relative amount of liquid recirculation within a compartment relative to the net liquid transport through a compartment can be controlled by varying the area of the liquid transport openings at the bottom of the compartment partitions relative to the area of the liquid recirculation openings at the bottom of the channel dividers, and also by varying-the weir overflow area.
The trays may be circular, rectangular, or other known shapes. Diameters in the range of 50 to 10,000 mm are contemplated. The vapor passages through each tray may be orifices, slots, tubes, valves, bubble caps, and the like. Hole diameters in the range of 0.5 mm to 20 mm are contemplated, and even larger for valve or bubble cap openings. Tray heights between 50 and 1000 mm are contemplated. Spacing between compartment partitions and channel dividers in the range of 5 to 500 mm is contemplated. Flooding limits are anticipated to be between 10% and 300% higher than for conventional sieve trays, dependent on vapor-liquid separator efficiency.
Prior art multistage tray contactors can be divided into three categories dependent upon where the liquid is introduced to each tray and where it is removed. The commonest is entry at one peripheral location and exit from the opposite periphery, with horizontal opposite-direction liquid flow across each tray. When this type of tray becomes large, a liquid concentration gradient forms across the tray. Unfortunately, the same size parameter which causes the liquid gradient also causes a vapor concentration gradient, e.g., prevents full mixing of the vapor, thus limiting the tray efficiency improvement factor.
The xe2x80x9cperipheral-to-peripheralxe2x80x9d category can also be configured for same-direction horizontal liquid flow. FIGS. 4 and 5 of U.S. Pat. No. 2,963,872 disclose one example of this, and FIG. 5 herein discloses another example. However, the full benefit of same-direction horizontal liquid flow is not obtained unless vapor mixing is prevented. Hence, this invention extends to adding transverse partitions at least in the vapor space to any of the same-direction horizontal liquid flow configurations.
The second category of liquid entry to and exit from the tray is center-to-center. U.S. Pat. No. 4,032,410 discloses one center-to-center configuration, with opposite direction liquid horizontal flow on each adjoining tray. FIGS. 7, 13, and 22 herein disclose new approaches to achieving the center-to-center flow configuration. They have the advantage of much larger weir length, larger downcomer size, and greater downcomer height, thus accommodating higher liquid loadings. Also, very importantly, a simple construction is disclosed which achieves horizontal same-direction liquid flow, including a concentration gradient, in the center-to-center configuration.
The third category is mixedxe2x80x94either center-to-periphery or periphery-to center. Examples of this are found in xe2x80x9cMultiple Downcomerxe2x80x9d trays, in the Oldershaw configuration, and in FIG. 1 herein.
The new center-to-center configurations disclosed herein have another advantage in addition to preserving reasonable weir lengths and providing large downcomer area-and height without using as much of the column cross section as conventional designs. The additional advantage is a clear periphery or circumference. This facilitates incorporating heat exchange into the column so as to reduce the energy requirement. One approach is to exchange heat with fluid in an annular space outside the tray wall. This is effective, since the entire wall circumference is wetted with two-phase froth. However, the surface-to-volume effect makes this less effective at larger diameters. Another approach is to mount heat exchange tubing inside the column on each tray, suspended in the froth. This can be done with simple vertical-axis coils on each tray. However, that doesn""t take advantage of the temperature gradient on each tray. In order to do that, a horizontal-axis conforming array coil is used, as illustrated in FIG. 13. Two or more coils of approximately mirror image are used on each tray, to accommodate the liquid which flows in two mirror image 180xc2x0 segments around each tray, thereby providing fully counter-current heat exchange. For other tray designs wherein there is only a single liquid path, e.g., a xe2x80x9cringxe2x80x9d tray, there need only be a single conforming coil for the entire periphery. There again, the transverse partitions which prevent vapor mixing are a key addition. The transverse partitions also provide convenient mounting and support points for the coils, whatever type of coil is used.
In summary, all three of the major tray column limitationsxe2x80x94column height, column diameter, and column energy demandxe2x80x94are improved by a single new configuration with mutually compatible and synergistic features: center-to-center liquid transport in the same horizontal direction with partitions to prevent vapor mixing; conforming heat exchange coils which are supported by the partitions; and channel weirs plus down comer zones for same-direction vertical liquid flow. Furthermore, for situations wherein all three limitations are not significant, appropriate sub-combinations of the newly disclosed configuration are adaptable to improve any single limitation or any combination of two of the three. For example, when an existing column is to be retrofitted, if a higher capacity is the only improvement called for, then only liquid recirculation (same-direction vertical flow) may be provided. If a sharper separation (purer products) is the only needed improvement, then only same-direction horizontal flow plus partitions may be called for. And if reduced energy demand is the only needed improvement, then center-to-center feed plus heat exchange coil may be the only improvement used.